1. Field of Invention
The present invention relates to a controlling device and, in particular, to a fan rotary speed controlling device.
2. Related Art
The fans are usually used for heat dissipation. They can be installed in the electronic products for extending the lifespan and cleanness of the products, or they can be used for decreasing the temperature of some environment, so that fans have become an indispensable kind of electronic products in current life.
In the conventional fans, the fan rotary speed is usually controlled by a pulse width modulation (PWM) signal and the duty cycle of the waveform is used to control the ratio of ON-OFF of the switching element (e.g. a transistor) in a unit time, thereby controlling the current value of the load. Accordingly, the small signal can directly control the fan rotary speed instead of changing the inputted voltage of the fan. FIG. 1 is a schematic diagram showing a conventional fan rotary speed controlling device 1. As shown in FIG. 1, an external voltage V1 (power source) is applied to the fan rotary speed controlling device 1 for driving it. In addition, a PWM signal S1 is inputted to the controlling circuit 11 for controlling the rotary speed of the fan motor 7.
However, the conventional fan rotary speed controlling device is easily affected by the changes or unstable of the inputted external voltage and the difference of air resistances, which may cause the variation of the fan rotary speed under the same PWM duty cycle. Therefore, it is an important subject to provide a fan rotary speed controlling device that can provide stable rotary speed without being interfered by air resistances.